This invention relates to abrasion-resistant cast irons suitable as materials for machine parts which require abrasion resistance, such as piston rings, cylinder liners, cam shafts, or tappets.
As is well known, there are various kinds of abrasion-resistant cast irons, and those now in use are classified into white cast iron and mottled cast iron which are high alloy cast irons and gray cast iron which is a low alloy cast iron. Usages of the white iron and gray iron are clearly differentiated from the standpoint of the mode of wear and abrasion. The abrasion-resistant cast irons of this invention belong to the gray iron, but also include mottled iron.
The gray iron, as is well known, consists of a matrix structure composed of pearlite, ferrite, or martensite, etc., graphite flakes, carbides, and others. Various investigations have been undertaken as to the effects of the graphite structure and the matrix structure on abrasion-resisting characteristics, and agreement is seen in the results obtained. Researches have also been conducted widely on the effects of the chemical composition of the gray iron on mechanical properties as well as abrasion resistance. But the wearing phenomenon is so complicated that its cause is still unknown in many respects.
The present inventors have found that boron (B) used in very small amounts leads to the formation of a carbide having high hardness which serves to increase abrasion resistance; that steadite (Fe.sub.3 P eutectic) observed in phosphorus-containing cast irons contains boron; and therefore that high hardness special steadite composed of Fe-C-P-B serves to increase abrasion-resisting characteristics.
Cast irons containing phosphorus have been used for castings having small thickness because of their improved fluidity. They also have found wide use as low-cost abrasion resistant cast irons because steadite is of relatively high hardness and is effective for increasing abrasion resistance.
As is seen in boron steel, it has been the practice to include a very small amount of boron in steel. Furthermore, although based on quite a different basic concept, the addition of boron to cast iron is disclosed in U.S. Pat. No. 2,046,912 directed to hard cast iron alloy, U.S. Pat. No. 2,390,594 directed to heat resistant cast iron, and U.S. Pat. No. 2,630,382 directed to cast iron filler metal.
As graphite present in the structure of cast iron acts as a solid lubricant, it exerts a very great effect on abrasion-resisting charactristics. On the other hand, it is known that graphite in flaky form gives the best result in affording abrasion resistance. Although graphite acts as a solid lubricant, too large an amount of it will result in a reduction in the strength of cast iron. For this reason, the amount of graphite is naturally limited. The carbide is also very effective for abrasion resistance because it has high hardness, high melting point and high strength, and possesses great load-bearing ability. Like graphite, excessive amounts of the carbide cause brittleness to cast iron, and reduce its workability, and hence, there is a limit to its amount.
Internal combustion engines have recently been operated at increasingly higher engine speeds and with increasingly higher outputs, and their component parts, such as piston rings or cylinder liners, are required to have both a high level of scuffing resistance and abrasion resistance. However, conventional internal combustion engine parts have a critical planar pressure, with regard to scuffing resistance, of about 25 kg/cm.sup.2, and an amount of wear of about 0.046 mg/cm.sup.2 .multidot.km with regard to abrasion resistance, and are still unsatisfactory.